Kavli Affiliate: Kohei Inayoshi
| First 5 Authors: Haojie Hu, Kohei Inayoshi, Zoltan Haiman, Luis C. Ho, Ken Ohsuga
| Summary:
The James Webb Space Telescope (JWST) has revealed low-luminosity active
galactic nuclei (AGNs) at redshifts of $zgtrsim 4-7$, many of which host
accreting massive black holes (BHs) with BH-to-galaxy mass ($M_{rm
BH}/M_{star}$) ratios exceeding the local values by more than an order of
magnitude. The origin of these overmassive BHs remains unclear but requires
potential contributions from heavy seeds and/or episodes of super-Eddington
accretion. We present a growth model coupled with dark matter halo assembly to
explore the evolution of the $M_{rm BH}/M_{star}$ ratio under different
seeding and feedback scenarios. Given the gas inflow rates in protogalaxies,
BHs grow episodically at moderate super-Eddington rates and the mass ratio
increases early on, despite significant mass loss through feedback. Regardless
of seeding mechanisms, the mass ratio converges to a universal value $sim
0.1-0.3$, set by the balance between gas feeding and star formation efficiency
in the nucleus. This behavior defines an attractor in the $M_{rm
BH}-M_{star}$ diagram, where overmassive BHs grow more slowly than their
hosts, while undermassive seeds experience rapid growth before aligning with
the attractor. We derive an analytical expression for the universal mass ratio,
linking it to feedback strength and halo growth. The convergence of
evolutionary tracks erases seeding information from the mass ratio by $zsim
4-6$. Detecting BHs with $sim 10^{5-6}~M_odot$ at higher redshifts that
deviate from convergence trend would provide key diagnostics of their birth
conditions.
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